1. Field of the Invention
The present invention relates to a system for detecting a trouble occurrence location of an electric power cable line by using a distribution type temperature sensor and particularly a Raman backscattering optical fiber distribution type temperature sensor and, more particularly, to a structure in which an optical fiber of a temperature detector of a distribution type tempe-rature sensor is laid along an electric power cable line.
2. Description of the Related Art
Recently, as a system for detecting an occurrence location of a trouble such as a ground-fault in an electric power cable line (a short-circuit between an electric power cable line and the ground due to a breakdown of the insulator of the electric power cable line), a system using a Raman backscattering optical fiber distribution type temperature sensor has been developed as disclosed, for example, in Published Unexamined Japanese Patent Application No. 1-267428. More specifically, the Raman backscattering optical fiber distribution type temperature sensor can measure a temperature distribution in the longitudinal direction of an optical fiber of its temperature detector. When the optical fiber is laid along an electric power cable, a position on the electric power cable line where a temperature rises due to a trouble such as a ground-fault is detected to identify the trouble occurrence location.
A principle of measuring a temperature distribution by the above-described Raman backscattering optical fiber distribution type temperature sensor is as below. When a light is incident into an optical fiber, the light is scattered due to the small fluctuation of a refractive index in the optical fiber, absorption, or re-emission of a light by molecules, atoms of the optical fiber. There are as the scattered lights a Rayleigh scattering light having the same wavelength as the incident light and a Raman backscattering light having a different wavelength from the incident light. The latter Raman backscattering light is generated by the thermal vibration of molecules, atoms of the optical fiber, and its intensity depends largely upon its temperature. Therefore, when a pulse light having a specific wavelength (normally a laser pulse) is used as the incident light, the delay of a period of time until the light is returned as the scattered light and the intensity of the Raman backscattered light are detected, the temperatures of the positions of the optical fiber in the respective directions can be measured.
In order to lay an electric power cable line, it is common to sequentially connect electric power cables of a predetermined length (unit cable). Therefore, joint portions are always present along the electric power cable line. On the other hand, to perform maintenance of the electric power cable, it is rare to achieve maintenance and supervision of the entire electric power cable line of a long distance. It is common to divide the electric power cable line into a plurality of maintenance sections and to perform the maintenance of the respective maintenance sections by another sections or maintenance duty persons in charge. In this case, it is normal to place a boundary (maintenance boundary point) between the maintenance sections at the joint portion of the electric power cables.
Therefore, as a conventional method for laying a detecting optical fiber along an electric power cable line to actually apply an electric power cable line trouble occurrence location detecting system using the above-described optical fiber distribution type temperature sensor, a method as shown in FIG. 1 is employed.
In FIG. 1, an electric power cable line 1 has a plurality of unit cables 3A, 3B, 3C and 3D connected in series through joint portions 2A, 2B and 2C. The electric power cable line 1 is divided into a plurality of maintenance sections 4A to 4D at the intermediate positions of the joint portions 2A to 2C as boundaries at the respective unit cables 3A to 3D. Independent optical fibers 5A, 5B, 5C and 5D are respectively laid along the unit cables 3A, 3B, 3C and 3D at the unit cables 3A to 3D of the maintenance sections 4A to 4D. The ends of the optical fibers 5A to 5D are respectively connected to distribution type temperature sensor processing units 6A to 6D, which are, in turn, connected to a host computer 9.
In such a system as described above, a temperature peak position on the electric power cable line, i.e., an occurrence location of a trouble such as a ground-fault of the electric power cable line can be detected at each of the respective unit cables, i.e., the respective maintenance sections.
Since the sections or persons in charge of maintenance and supervision are different in the respective maintenance sections along the electric power cable line as described above, it is necessary to know where a trouble is generated in the maintenance sections, and it is particularly necessary to accurately detect where a trouble occurs in the adjacent maintenance sections in the vicinity of the boundary between the maintenance sections. On the other hand, the boundary between the maintenance sections is frequently disposed at the joint portion of the unit cables. The occurrence frequency of a trouble such as a ground-fault at the joint portions of the unit cables is remarkably higher than that at the normal portion of the electric power cable. Therefore, it is strongly desired to improve the detecting accuracy of the trouble occurrence location of the joint portion of the unit cables.
The Raman backscattering optical fiber distribution type temperature sensor can considerably accurately detect the temperature peak position due to its characteristics and an electric power cable line trouble occurrence location detecting system using the temperature sensor can considerably accurately detect the occurrence location of a trouble such as a ground-fault. However, it is not yet sufficient to accurately detect where the trouble occurrence location belongs to the section of which side in the vicinity of the boundary between the maintenance sections as described above, and it is not yet sufficient from the view of accurately detecting the trouble occurrence location at the joint portions of the unit cables having high trouble occurrence frequency.